Fluctuations in mineralogical composition of banded gneisses along a profile, 230 mm of length, are given. The rocks are shown to have suffered a potash metasomatism, giving rise to light-coloured layers or veins rich in potash feldspar and surrounded by rims of biotite and epidote. The influence of tectonical movements upon the development of different kinds of light-coloured layers, veins, and irregular bodies is pointed out.